The invention relates to a turbine for an exhaust gas turbocharger including a turbine wheel with a number of turbine blades, a first radially extending inlet flow channel and a second inlet flow channel which extends semi-axially to the turbine wheel and variable flow guide means arranged in the first inlet channel to permit motor braking operation of the internal combustion engine on which the turbocharger is installed.
Such turbines are known for example from DE 34 27 715 C1, DE 39 08 285 C1, DE 43 30 487 C1 or DE 196 15 237 C2. Exhaust gas is admitted to the turbine wheels of these turbines by way of exhaust gas admission channels, which extend radially and also semi-axially wherein the radially extending flow channels are closed during motor braking operation by a variable guide vane structure whereby the turbines are then operated practically as semi-axial turbines. With these so-called turbo-brakes, a substantially increased braking power can be achieved in comparison with the normal suction engines.
Problematic for these turbines and for turbochargers, which include such turbines, is the good efficiency of the turbine under normal operating conditions since the high motor braking power needed results in an excessive speed of the turbine wheel. Such an excessive turbine wheel speed may result in damages to the compressor and the turbine wheel. Therefore, in practice, the variable flow guide structure at the entrance to the radial flow channel is always kept open to a certain degree in order to reduce the pressure in the semi-axial flow channel. In this way, however, the desired theoretically achievable motor braking power cannot be achieved as it is possible with the blowing down of gases or throttling of gases by an additional throttle valve.
However, the above measures generally result in a cost increase of the turbines, in increased space requirements and in a higher risk of failure.
It is therefore the object of the present invention to provide a turbine for an exhaust gas turbocharger, which is simple in design, yet provides for high motor braking power.
In a turbine for an exhaust gas turbocharger including a turbine wheel with a number of turbine wheel blades supported in a housing, which has a first radial inlet flow channel including a variable guide vane structure for guiding exhaust gas radially onto the turbine wheel, and a second, semi-axial, flow channel for guiding exhaust gas semi-axially to the turbine wheel, the turbine wheel blades are arranged and sized so as to define between circumferentially adjacent blades a flow passage with a flow cross-section which has a minimum at its inlet end.
The cross-section between adjacent turbine blades in the semi-axial inlet channel is designed so as to provide for a minimum flow area at the semi-axial entrance of the second flow channel to the turbine wheel. The flow passage for the in-flowing exhaust gases is so narrow that, at a certain relatively high turbine wheel speed, preferably near the limit speed or slightly below that speed, the gas flow is blocked. This is achieved as the incoming exhaust gas flow reaches the speed of sound in the narrowest cross-section. A further increase of the turbine wheel speed is then no longer possible. As a result, the operating efficiency of the turbine becomes lower whereby, on one hand, the turbine speed is not further increased and, on the other hand, a higher braking power with a higher energy flux is achieved.
However, the design of the turbine wheel in accordance with the invention does not detrimentally affect the normal operation of the turbine wherein the engine power output is increased since under normal operation the exhaust gas pressure is substantially lower so that the above-mentioned blocking conditions will generally not occur.
It is noted in this connection that, with the circumferential speed of the turbine wheel, the effective angle of attack to the turbine wheel changes. Consequently, the effective inlet flow cross-section to the turbine wheel is speed-dependent. At high turbine wheel speeds, the effective inlet cross-section is substantially reduced so that the blocking conditions referred to above become earlier effective.
With regard to the desired deterioration of the efficiency, it is particularly advantageous if the open flow cross-section between two blades of the turbine wheel in the inlet area of the second flow channel is smaller than the outlet cross-section between the wheel blades of the turbine wheel
In practice, particular good conditions exist when, in the semi-axial inlet area of the second flow channel, the inlet cross-section between two adjacent blades of the turbine wheel is less than three times the exit cross-section between two adjacent guide vanes. It is furthermore advantageous if the inlet cross-section between two adjacent turbine wheel blades at the semi-axial inlet of the second flow channel is equal to, or larger than, the outlet cross-section between two adjacent guide vanes of the guide vane structure.
Advantageous embodiments of the invention will become more readily apparent from the following description on the basis of the accompanying drawings.